1. Field of Invention
This invention relates to the field of nuclear medicine, and in particular to nuclear medicine imaging systems.
2. Description of the Related Art
Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images that show the function and anatomy of organs, bones or tissues of the body. During one form of imaging, so-called Single Photon Emission Computerized Tomography (SPECT) imaging, radio-pharmaceuticals may be introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radio-pharmaceuticals produce gamma photon emissions that emanate from the body.
One or more detectors are used to detect the emitted gamma photons, and the information collected from the detector(s) is processed to calculate the position of origin of the emitted photon from the source (i.e., the body organ or tissue under study). The accumulation of a large number of emitted gamma positions allows an image of the organ or tissue under study to be displayed.
In another form of imaging, called transmission imaging, radiation from a radioactive source passes through an object and is collected by a detector. Different structures within the object collect or retard the progress of radiation differently, producing images representative of those structures at the detector.
If a radioactive line source is used, it may be swept across the detector's field of view, behind the object to be viewed. The detector then receives the transmitted radiation from the line source as it passes successive locations of the object. If the detector is rotated around the object to get a 360° image, the line source may be placed parallel to the axis of rotation of the detector, and offset somewhat. As the detector moves around the object, the relative motion between it and the line source will sweep the line source across the detector's field of view, behind the object.
If, on the other hand, the detector translates along the length of the object, the line source may be placed normal to the direction of translation. As the detector moves down the object, the relative motion between it and the line source will sweep the line source across the detector's field of view.
If both rotation and translation are desired, however, these two line source orientations are mutually incompatible. Neither orientation will suit both detector rotation about the direction of translation, and translation of the bed itself.
If, e.g. the line source is normal to the direction of translation, its length won't sweep across the field of view of a rotating detector. Conversely, if the line source is parallel to the direction of translation, its length won't sweep across the field of view of a translating detector.
In this case, either two line sources are used, one parallel to and the other normal to the direction of translation, or else rotation and translation are performed in separate passes. The line source is placed parallel to the axis of rotation and a rotation pass is made, and then the line source is rotated 90° to the axis of rotation, or normal to the direction of translation, and a second, translation pass is made. If the axis of rotation of the detector is parallel to the direction of translation.
Proper imaging requires that the detector be relatively stationary with respect to the line source to eliminate or minimize distortions. Such distortions can be caused, for example, by lack of a uniform response to incident radiation over the entire area of the scintillation detector surface, by non-linear responses to incident radiation by different photo-multiplier tubes arrayed over the scintillation crystal, and by variations in the energy window defining the range of photon energy levels of a scintillation interaction or “event” that will be accepted as contributing to the image.